"...Once you leave active duty, it's generally understood that that's it—your military days are over, and a life in the civilian world awaits you.
But what if some major world event happened? Could you be recalled back into active duty service?
Rules for Military Retirees
Retirees (those who spend at least 20 years in the military and draw retired pay) can be recalled to active duty for life.
However, the policy established in "DoD Instruction 1352.01 - Management of Regular and Reserve Retired Military Members" makes the recall to active duty unlikely for those who have been retired for more than five years, and those over age 60."
Read more details about the IRR to Active Duty possibilities at The Balance Careers